Process and apparatus for purification of crystals



" June 16, `1959 w. R, EDDY ETAL PROCESS AND-APPARATUS FOR PURIFICATIN OF CRYSTALS 2 Sheet s-Sheet 1 Filed Nov. 14, 1955 jNVENTRs W. R. ED DY 4 D. L. MCKAY ATTORNEYS June 16, 1959 w. R. EDDY ETAL 2,890,962

PROCESS AND APPARATUS FOR PURIFICATION OF CRYSTALS AT TORN EYS tion.

United States Patent O" PROCESS AND APPARATUS FOR PURIFICATION oF CRYSTALS William R. Eddy and Dwight L. McKay, Bartlesville,

Okla., Eassignors to Phillips Petroleum Company, a corporation of Delaware Application November 14, 1955, Serial No. 546,732

` 9 Claims. (C1. s9zos) in said liquid removal Zone, melting crystals in said melt-` ing zone, withdrawing pant of the melt as product, and forcing another part of the melt in a direction countercurrent to the movement of crystals in said reux zone. This process is generally applicable to the separation of at least one pure component from any mixture which is resolvable into its components by fractional crystalliza- For example, the process can be used for the concentration of fruit juices, Vegetable juices and other materials which comprise aqueous solutions which can be concentrated by the formation and removal of ice crystals. The process is also of great value in the resolution of non-aqueous mixtures, one example of such an application being the separation of para-xylene from a mixture thereof with the other xylene isomers and ethyl benzene; and another example being the separation of methyl vinylpyridine from mixtures of same with methyl ethylpyridine.

The purity of the product from a crystal puriiication process as above described is governed by the effectiveness of the reiiux which passes in countercurrent relationship to the passage of the crystal mass. The maximum rate of ilow of reflux is principally governed by the ability `of the reilux material to ilow through the crystal bed without channeling and without eroding the crystal bed. The reflux material, which is a melt of purified crystals, should pass through the crystal bed and, as a result of contact with the cold crystals, freeze, and in freezing displace occluded mother liquid in a direction countercurrent to the movement of crystals through the melting zo e.

1various attempts have been made to melt the crystal bed by reintroduction of hot melt to the crystal melting section so as to avoid the high skin temperature of heating elements which are in direct heat relationship with the crystal bed so as to provde a suiiicient quantity of heat to melt the required amount of crystals for adequate reilux. The problem which has accompanied the use of crystal melt `as a heating means is the channeling and erosion ofthe crystal bed resulting from the high velocit-y of the iiow of crystal melt;`

The present invention provides a process and arrapparatus for effecting a separation of the type described at higher" throughput rates, improved stability and ease of operation, and improved heat distribution by a novel means for providing a high rate of reintroduction of hot melt to the crystal melting zone. In addition, products pf high purity are obtainable over long periods of oper-A 2,890,962 Patented June 16, 1959 i ce ation because substantial uniformity of crystal concentration in the crystal bed can be maintained throughout the operating period of the process. According to the present invention hot crystal melt is reintroduced into the crystal bed through a permeable member which is capable of delivering a large amount of crystal melt at low velocity and with uniform distribution with respect to the surface of the crystal bed exposed thereto.

Further, in accordance with this invention, in a process wherein solids are countercurrently contacted with the liquid in a purification Zone, the solids are supplied continuously to said zone and the liquid is supplied continuously and distributed uniformly throughout the solids.

Also in accordance with this invention, there is provided, in an apparatus comprising a puriiication chamber, liquid-solid separation means in said chamber, melting means positioned in one end of said chamber, means for moving solids to said separation means and thence toward said melting means, means for removing melt from said chamber, and means vfor removing liquid from said separation means, the improvement which comprises means for returning a portion of said melt through said melting means evenly distributed over the surface of lsaid melting means and into contact With said solids and means provided in said melting means for tlow of melt from said bed to said means for removing melt from said chamber.

According to one embodiment of the invention, a mass of crystals to be purified, together with accompanying liquid, is passed into a puriication chamber containing a filter which is coextensive with the wall of said chamber and positioned at an intermediate part thereof, then through `a reilux zone toward a heating means positioned in the downstream end, with respect to flow of crystals through the chamber, which heating means comprises a double walled member with the side toward the crystal bed being made of porous, sintered metal, and the other` wall being made of nonporous metal, said heating means containing a plurality of ducts extending through both walls of the double walled member. The melt is withdrawn, in part, as purilied product, and forced, in part,

into the interior of the double Walled heating member so as to flow through the porous sintered metal wall of the heating member, and melt the approaching crystal bed uniformly across its longitudinal axis. Thus, according :to this invention, as the crystals `are supplied to the puriiication zone, the reflux or crystal melt is caused to flow uniformly into the moving bed of crystals from the opposite direction and to melt the crystals of said moving bed at a high rate without erosion or channeling.

The invention `is equally applicable to the various types of purification columns such as the piston type purification column as exempliiied by Schmidt, Reissue 23,810

(1954), the pulse type column `as disclosed and claimed in application Serial No. 494,866, filed March 17, 1955, by R. W. Thomas, the pulse type column disclosed and claimed in application Serial No. 546,101, tiled November l0, 1955, by P. S. Stallings, lr., and the agitated type column disclosed and claimed in application Serial No. 375,850, led August 24, 1953.

An important feature of the invention lies in the fact that materials which are heat sensitive such as dimethyl phthalates can be successfully purified without danger of decomposition which would otherwise result from the high skin temperatures necessary with the use of conventional heaters.

The drawing illustrates a preferred embodiment of this invention and it is to be understood that conventional equipment such as pumps, etc., and conventional iittings such as valves, and conventional control instruments such.

as temperature and pressure control instrtunents,are.notY

shown butwill` readily be supplied by those skilled in theart:

Figure l is an elevationalgfview, partly in section, of fractional crystallization apparatus. illustrating the presont invention;

FigureZis across-sectional illustration of the novel heating elementof' this invention; and

Figure 3 is a plan view of the heating element ofFigure 2;

Corresponding numerals indicate corresponding parts inl each-fof `the-three figures.

The apparatus, of Figure l comprises freezing or crystallizingmeans A, a filteringrmeans B, conveyor zone C, a-secondifilteringlmeans D, a refiux zone E, and a melting` zone' F;

AL feed stream comprising, for example, a mixture of dimethyl phthalates is introducedto chiller lll' by means of'l line 11; Chiller lll" can be a conventional scraped surface chiller comprising anv inner and outercylinder spaced'apart so asto form an annular space therebetween forcirculation of a refrigerantwhich can be admitted through line Irland removed through line 13. The chiller is operated ata temperature which will freeze at-leastY a portion of the component to be separated from the-mixture. The resulting slurry of crystals and mother liquor is passed from the chilling section to the filtering means B which can be a conventional rotary filter. Motherliquor is removed from the filter and discarded fromp the system through line 14and'the crystals passed through conduit 15V to conveyor 16 and are introduced into crystal purification column 17 by suitable means such as by the action of screw conveyor 1S. means'such as a piston canalso be utilized.

The crystals are compacted and moved' toward the melting section F ofcolumn 17 by means of piston 19 which is actuatedk by flow of fluid in lines 21 and 22 to4 cylinder 23. Mother liquor is removedfrom filtering section D which can be perforated section 24 of column 17 enclosed by a bustle ring 25. Mother liquor is passed by means of line 26 and controlled by valve 27 to a reprocessing step. The mother liquor can be returned to feed line 11 and thus recycled to the process.

As the mass of crystals is moved toward the melting zone F a flow of hot crystal meltis forced through the porous or perforate metalsurface 28 of heater 29. A portion of the crystal melt is refrozen in the crystal mass andthe remaining crystal melt is returned through heating means 29" by way ofthe passage ways 31 to the bottom of column 17. Crystal melt is withdrawn from the bottom of the column 17 through line 32. The crystal melt in line 32 is pumped by means of pump 34 through filters 35 and 35 and heater 36 and thus returned to heating means 29. A predetermined portion of the crystal melt is withdrawn through line 38 as` the product of the process.

The valves ofline 33 are normally closed. This line is utilized to increase the flow of hot liquid through the heater 29 `when additional heating is required.

Filters 35 and 35 are advantageously operated alternately; one being in use while the other is` being recharged. The filters guard `against passage of solids whichvwould plug the pores of the porous metal.

Figure 2 shows the details `of the heating means 29.

Figure 3 shows a plan View of the heating means` 29.

The heater Z9 is shown as a ydouble walled cone, however, the heater can be a flat plate, an inverted cone, a concave or convex segment of a sphere, or any other configuration wherein the heater occupies` substantially the entire cross-section of the column with passage ways provided for flow of crystal melt and with the porous Wall of the heater in contact with, or facing, the crystalV Other 4. termined by the particle size of the metal powder. The pore size can vary within a wide range ofsizes; however, it is usually preferred to employ a pore size in the range of lO to 165 microns. The thickness of the porous metal wall can also vary within a wide range and the thickness is usually in the range of 1/16 to s inch.

The method and apparatus `of this invention can be advantageously utilized in practically any system towhich fractional crystallization is applicable. This invention is applicable to separations in many multi-component sys-.- tems, the components of which have practically the same boiling point and are, therefore, difficult to separate by fractional distillation, or to mixtures which have diverse boiling points but which `form azeotropesor are. heat sensitive. The effective separation, of components of such mixtures may be made from systems where the concentration of one component is relatively high, or where the concentrations of the components are about equal. One particular advantageous application ofthe process lies in the purification of a component of, say 15 to 25 percent purity, so as to effect a purity upwards of 98 percent. In order to illustrate some of the systems' to which the invention is applicable, the following com-` pounds are grouped withrespect to their boiling points.

Group A B.P., F.P.,

Benzene 5. 5'V n-Hexane 69 94.

79. 9 87. 5z Methyl acrylate 80. 5 1,3Cyclohexadiene 80. 5 98 2,4-Dtrnethy1pentane 80. 8 123. 4 i 2,2,3-Tr11uethy1butane 80. 9 25 Cyclohexane 81. 4 6. 5 Acetonitrile. 82 42 Cyelohexene 83 103. 7. 2-Methylhexane 90 119 S-W/Isthylhexane 89. 4` 119. 4'

Group B B.P., F3.,

Methyl cyc1ohexane 100. 3 f 126. 3 Oyclohexane. 81. 4 6. 5 n-Heptane 98. 52 90. 5 2,2,4-Trimetl1ylpentane 99. 3 107. 4 Nitromethane. 101 29 p-Dioxane 101. 5 11.7 2Pentanone 101. 7 77. 8l 2-Methy1-9fbutanol... 101. 8 11.79, 2,3-Dimethylpentane. 89. 4 B-Ethylpeutaue 93. 3 94; 5

Group C B2., F.P.,

C. D G;

Toluene Methylcyelohexa 2,2,3.3Tetramethyl butano 2,5-Dimenthy1hexaue- 2,4-Dimethylhexane 2,3-Dmethylhaxanp 3,4-Dirnethy1hexaue. 3-Ethy1-l-methylpentana A 3-Ethyl8methylpeutaue Group .D B.P., O. F P C.

Anllinn 184.4 5.2. Toluene 110. 8 95 Ben Wup 80. 0 5. 5

Group E B P C. F.P.,.O.

Carbon Tetraehlorlde 77 22:8 Ohloroform 61 6355 OS 46.3 108.6 Aootnnn 56 5 95 Orthox lene 144 27.1

Metafxyylpne 138. 8 -47. 4

Paraerylena 138. 5 13. 2

Group G B.P., C F.P., C

Ortho-eymene 175, 73. 5

Metafcymene 175. 7 -25 Para-eymene 176. 0 73. 5

` Group H B.P., C. F.P., C.

Dimethyl phthalate 282 5.

Dimethyl isophthalate 124 G7 Dimethyl terephthalate 288 140. 6

Group I i B.P., C. F.P., C

Ortho-nitrotoluene i 222. 3 ca -1g.

Metamitrotoluene 231 15. 5

Para-nitrotoluene 238 51. 3

Systems consisting of any combination of two or more of the components 'Within any one of the groups may be separated by the process of the invention, as well as systems made up 4of components selected from different groups; for example, benzene may be separated from a benzene, n-hexane or n`heptane system in which `the benzene is present in an amount greater that the eutectic concentration. In the same manner, para-xylene may be readily separated from a mixture of paraand metaxylenes or from para, meta, and ortho-xylenes. Benzene may also be separated from admixture with toluene and/or aniline. Multi-component systems which may be effectively separated so as to recover one or more of the components in substantially pure form include 2,2- dimethylpentane,` 2,4-dimethylpentane, 2,2,3-trimethylbutane, methyl cyclohexane, 2,2,4-trimethylpentane, and carbon tetrachloride, chloroform, and acetone. The invention is also applicable to the separation of individual components from a system of cymenes and a system including the xylenes.

This invention can also be utilized to purify naphthalene, hydroquinone (1,4-benzenediol), para-cresol, paradichlorobenzene, and such materials as high melting waxes, fatty acids, and high molecular weight normal paralins. The system can also be used to separate anthracene, phenanthrene, and carbazole. Furthermore, the system can be used to separate durene (1,2,4,5tetra methylbenzene) from C10 aromatics. In cases where the material to be puritied has a relatively high crystallization point, the impure material is heated to a temperature at `which only a portion of the constituents are ina crystalline stateV and the resulting slurry is handled at such a temperature that operation is as described in connection with materials which crystallize at lower temperatures.

The invention is also applicable to the concentration of food products. In the preparation of such concentrated foods, the process consists generally of the removal of water from such products. One special class of foods which can be concentrated in this manner is that of fruit juices, such as grape, pineapple, Watermelon, apple, orange, lemon, lime, tangerine, grapefruit, and the like. Beverages, such as milk, wine, beer, coffee, tea, and various liquors, can also be concentrated in such a process. The process is also applicable to the concentration of vegetable juices.

A better understanding of the invention can be obtained by reference to the following example which is intended to exemplify but not unduly restrict my invention.

' Example A feed mixture comprising 17.0 Weight percent para? xylene, 17.5` weight percent` ortho-xylene, 33.4 weight percent meta-xylene, 27.5 weight percent ethyl benzene and 4.6 percent toluene is supplied to the cooling zone A at the rate of 857 gallons per hour. Liquid material recovered from column 17 through separation means 25 and line 26 contains about 40 weight percent para- Xylene. This stream is also supplied` to the cooling zone at a rate of 128 gallons per hour so that the total feed to the cooling zone is 985 gallons per hour. That material is cooled to a temperature of 70 C. with the resultant formation of about 15 weight percent solids. The slurry of mother liquor and crystals is passed to the iiltering zone B and 769 gallons per hour of iirst stage mother liquor containing about 6 Weight percent para- Xylene is removed from the filtering zone through line 14. The crystals recovered from the filtering zone comprise the feed to the crystal puriication column 17 and are introduced into the purification column in the form of a slurry of crystals and mother liquor resulting from melting the crystals at about 1 C. and cooling the resultant melt to about 10 C. The feed to column 17 is 216 gallons per hour of a slurry containing 40 weight percent solids.

Compacting piston 19 is operated by a conventional power source such as a hydraulic piston and cylinder so as to move the crystal mass through crystal purification column 17 toward the heating means 29 in melting zone F. Heat is added to the melting zone F in the end portion of the puriiication column by reintroduction of hot crystal melt Ato heating means 29 as hereinafter described so as to maintain the desired product rate. Second stage mother liquor is withdrawn from the column through separation means 25 and 4line 26 at the rate of 128 gallons per hour and at a temperature of about 10 C.

, Crystal melt is removed from purification column 17 through line 32 and product is taken through line 38 at the `rate of 88 gallons per hour and a temperature of about 27 C. containing about 99 weight percent paraxylene. The crystal melt not removed as product is returned to heating means 29 through line 37 after it is heated to about C. by means of heater 36. This hot melt is forced through the porous wall of heating means 29 into contact with the advancing crystal mass so as to provide the required amount of melting necessary to produce the reflux and the purified crystal product.

Variations and modifications are possible within the scope of the disclosure of the invention, the essence of which is the method of providing heat to the melting zone of a crystal puriiication column for melting the crystal bed which comprises passing hot crystal melt to said melting zone through a porous member so that a large volume of liquid is supplied at a low velocity and a device for accomplishing the method.

That whichis claimed is:

l. In apparatus wherein means are provided to move crystals toward a melting section of an elongated crystal puriiication chamber having a crystal inlet, 4a mother liquor outlet, a melting section and a purified crystal melt outlet, the improvement comprising rneans for heating a portion of melt removed from said melt outlet; pervious walled means positioned in said melting section for passing heated melt uniformly into contact with substantially the entire cross-sectional area of said crystals in said chamber; and means for passing heated melt from said melt heating means to said pervious: walled means.

2. Apparatus according to claim 1 wherein said pervious walled means comprises a double-walled cone occupying substantially the cross-sectional area of said elongated chamber with the upstream wall, with respect to the flow of crystals, pervious and the downstream wall impervious to the flow of melt and having a plurality of passage ways communicating with the upstream and 7, downstream portions of saidfchamber with respect to the movement of crystals therethrough. i

3`L Apparatus according to claim 1-wh'erein.said p ervious walled means comprises atdouble-Walledmember occupyingn substantiallyy the cross-sectional area of said elongatedLch'amb'er with the upstream wall," with respect tothe iiow` of crystals, pervious and the downstream Wall impervious to the .ilow of melt and havingV a plurality of passagev Ways therethrough in communication with theV upstream` and downstream portions `ofV said elongated chamber with respectito the. oW of crystals.

4.; In apparatus wherein means `areprovided so, as. to` move crystals toward *a heated section 4of anelongated crystal purication chamber having` a crystal inlet,. a mother'y liquor outlet, a heated-section, and. a, purified crystal melt outlet, the improvement comprisingmeans foreintroducing heat carryingfluidcomprising hot crystalmelt into theheated section ofth'e purification columnw and intocontact with the crystals therein in a volumeV suiicient to provide heat for meltingsaidcrystals and at'a velocity less than that requiredlto erode the crystal-bed; and meansrto distribute theheat carrying id^substantially uniformly across the cross-section of thecrystal bed:

5` Inthe'purification of crystals Whereincrystalsare moved through a purification zone into asmelting'zone incountercurrent'ow relationship to crysta1.melt,ithe improvement comprising the steps of. injectinga heatcarryingfiuid comprising hot crystallmelt into saidmelting'zoneas a plurality of streams distributed-uniformly across'isubstantially the entire cross-sectionof the meltingzone, ina quantity sucient to melt saidcrystals and at'ay velocity less than that required to erode said crystals; andmoving :said crystals into contact with said heatcarrying uid;

6. In an'apparatus for the purification of crystals comp rising a purification chamber, liquid-solid` separation means insaid chamber, perviousmelting means positioned in* one end of' said chamber, means for moving solids"to'saidseparation means and-thence towardtsaid melting' means, means for removing liquidtfrom 4saidiseparation means, and means for removingmelt fromsaid chambenthe improvement which' comprisesmeans for heating and returning a portion of said'melt through said pervious melting means evenly distributed substantially throughoutl the.cross-section of said purification chamber andinto contact with said solids;.and .means provided'through said melting means for flow ofmelt.fr`om said bed toV said/means for removingmelt fromsaid chamber;

7`. In the purication of crystals-wherein crystals are moved through a purification zone into a melting zone in countercurrent ilow relationship-to crystalrmelt, the improvementcomprising the stepsof removing a portionf'of'said crystal melt; heating saidfmelt tofatemperature above the melting pointof said crystals; and introducing said heated melt to said meltingr zone as a plurality` of streams distributed uniformly acrosssubstanti-ally` the entire cross-sectionzof.,said melting zone, in an amount t sufficient to melt said crystals and ata-velocity lesstthan that required toerode` theA crystals'. g

8; An apparatus for the-puriication of crystals comprising an elongated conduit, closed at both ends and having feed inlet means for introducing crystals and mother liquor at a first end and means for removing crystal'melt from the second end; liquid'pervious, crystal iinpervious filter means communicating between the. exterior of said conduit and .the interior ofssad conduit at azpoint intermediate the feed inlet means-andthe. melt removing. means; heating means positioned intermediate the lter meansy and melt removal means comprisingav double.- walled member occupying substantially the entire crosssectional area of said conduit-having the Wall facing the ilter means pervious to liquid and theopposite Wall impervious to liquid, said double-walled member having a plurality of passageways therethrough in communication with the upstream and downstream positions of said conduit with respect to the How of crystals; ,means for heating a portion of melt removed; and means for introducing heatedl melt -to said double-Walled member.'

9. An apparatus for the separation and purification of at least one component of a liquid mixture which comprises means, for` chilling; said: mixture.: soV asxto v form crystals of at'leastione component; .a-rstzlter-,means .for removing: mother,` liquor from theachilled mixture; an elongated conduitgclosed ateachend, having a-iirst crys-- tal meltioutletat one endaand'ameans-atithef otherend for moving: crystals Vtowardtsaidoutlet; frneans for` introducing. crystals from: said .rstlter means nto `said con-v duit at af point downstreamzfromfsaid. crystal'movingmeans with respect'to thefllow of crystals; asecond lter means communicating; betweenl the exterior of,v said i conduit and" theinterior; of: saidiconduitaat'a point downstreamV from said crystalf introducing means; heating meansl positionedl downstream: from'. said second. filter means comprising ai double-,Walledimember occupying substantially the entirencross-sectional area ofl said conduit, .having-the. upstream A Wall pervioustor` crystalA melt,l and having: a plurality of'passageways therethrough -communicating4 with the. upstream andl downstream `portions ofsaid conduit; melttinlet andi'second: melt 'outlet-means in said heating means; means for removing meltfrom said rst melt outletmeansandsrecoveringa` ffrstlportion of said-melt asprod-uctzof'the process; meanslfor heating, the .second l portion 1 ofi said melt yand introducing same tosaid meltiinlet;,and1means for'removingmelt from saidsecond outlet landreturningl same `to saidvmeans for heatingy said :second .portion of f said melt.'

References Citedin the le of 'this patent' UNITED.y STATES -PATENTS l 2,747,001. Weedman- May 22, `1956 2,765,921* Green Oct.y 9;' 1956 2,780,663 Gunness Feb. 5, 1957 

5. IN THE PURIFICATION OF CRYSTALS WHEREIN CRYSTALS ARE MOVED THROUGH A PURIFICATION ZONE INTO A MELTING ZONE IN COUNTERCURRENT FLOW RELATIONSHIP TO CRYSTAL MELT, THE IMPROVEMENT COMPRISING THE STEPS OF INJECTING A HEATCARRYING FLUID COMPRISING HOT CRYSTAL MELT INTO SAID MELTING ZONE AS A PLURALITY OF STREAMS DISTRIBUTED UNIFORMLY ACROSS SUBSTANTIALLY THE ENTIRE CROSS-SECTION OF THE MELTING ZONE, IN A QUANTITY SUFFICIENT TO MELT SAID CRYSTALS AND AT A VELOCITY LESS THAN THAT REQUIRED TO ERODE SAID CRYSTALS; AND MOVING SAID CRYSTALS INTO CONTACT WITH SAID HEATCARRYING FLUID. 